This forum is like the biggest collection of AMD sympathisers you'll find

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The answers you're being given, however, are correct. The AMD cards are better value for money and there aren't any overriding issues with the brand.
That said to answer your question: it is unlikely that the extra 2GB will make any difference over a 4GB version at 1080p. It comes down to the price difference between the two. If you can nab a 4GB version for a tiny bit more than the 2GB version then go for it. Otherwise, if it was me I'd get a 2GB version.
I'd only consider the 4GB if you're looking at definitely going SLI. If it's unlikely to happen at all then really you should look at 2GB.